The present invention relates to an electrophotosensitive material, which is used in image forming apparatuses such as electrophotographic copying machine, facsimile and laser beam printer, and an image forming method using the same. More particularly, the present invention relates to a single-layer type electrophotosensitive material which does not form a transfer memory image even when used in a reversal development type digital image forming apparatus, and a reversal development type digital image forming method using the same.
Recently, an organic photosensitive material has widely been used because it is easily produced at low cost as compared with a conventional inorganic photosensitive material and has advantages, for example, wide range of choice of photosensitive materials such as electric charge transferring material, electric charge generating material and binder resin, and high functional design freedom.
The organic photosensitive material includes a single-layer type photosensitive material wherein an electric charge transferring material (hole transferring material and electron transferring material) is dispersed in the same photosensitive layer, together with an electric charge generating material, and a multi-layer type photosensitive material comprising an electric charge generating layer containing an electric charge generating material and an electric charge transferring layer containing an electric charge transferring material, which are mutually laminated.
According to an image forming apparatus using an electrophotographic system, an image is formed by charging a photosensitive material (main charging step), exposing the photosensitive material to light to form an electrostatic latent image (exposure step), developing the electrostatic latent image with a toner in the state where a developing bias voltage is applied (developing step), transferring the toner image thus formed to a transfer paper (transferring step), and fixing the toner image. The residual toner on the photosensitive material is cleaned by a urethane blade (cleaning step) and the residual charges on the photosensitive material are erased by LED (charge neutralizing step).
The image forming apparatus utilizing the electrophotographic system includes digital and analogue copying machines, facsimile, and laser beam printer and, in particular, a reversal development system for developing using a toner having the same polarity as that of the charged voltage to be applied to the photosensitive material in the charging step has widely been used in a digital image forming apparatus.
When using the electrophotosensitive material in the reversal development type digital image forming apparatus, the transfer voltage to be applied to the photosensitive material in the transferring step is usually applied through a transfer medium (paper) without being directly applied to the photosensitive material, but is not applied when the transfer medium does not pass through the transferring step.
However, on-off timing of the transfer voltage is very difficult and the portion where the transfer voltage is applied directly to the photosensitive material is formed occasionally with respect to front and rear ends of the transfer medium. That is, the application of the transfer voltage starts before a transferring apparatus is covered with the front end of the transfer medium. Even if the rear end of the transfer medium passes through, thereby partially exposing the transferring apparatus, the transfer voltage is still applied continuously so that the transfer voltage is applied directly to the photosensitive material at the portion.
In case of the positive charging single-layer type photosensitive material, since the voltage to be applied by the transferring apparatus has a negative polarity, there arises so-called transfer memory wherein negative space charges are remained on the surface of the photosensitive material to which a negative voltage is applied. In general, since the single-layer type photosensitive material has the sensitivity in both polarities, negative space charges are erased in the subsequent charge neutralizing step.
However, in case the sensitivity of the positive charge single-layer type photosensitive material to a negative polarity is very poor (half-life exposure is very large), negative space charges are not erased sufficiently. If the photosensitive material is positively charged in the subsequent charging step, fall of potential is caused by an influence of space charges. In the developing step, a difference in sensitivity causes a so-called transfer memory image wherein the portion becomes black in the image.
In the image forming apparatus using the positive charging single-layer type electrophotosensitive material, the voltage to be applied by the transferring apparatus has a negative polarity and the voltage to be applied by a charging apparatus has a positive polarity. Therefore, ozone is evolved in the vicinity of the transferring apparatus, while Nox is evolved in the vicinity of the charging apparatus. Evolution of the gas tends to become severe in a large size image forming apparatus having a large voltage applied.
It has been known that exposure of the surface of the electrophotosensitive material to the gas lowers the charged potential of the portion, thereby causing image fog and black streaking.
Thus, an object of the present invention is to provide a single-layer type electrophotosensitive material which causes less transfer memory and no transfer memory image even when using in a reversal development type image forming apparatus, and is also superior in resistance to gases such as ozone, NOx and the like, and which is free from image defects even when using in a large size image forming apparatus.
Another object of the present invention is to provide a reversal development type image forming apparatus using the single-layer type electrophotosensitive material.
To attain the objects descried above, the present inventors have intensively studied and found such a fact that a transfer memory image is hardly formed even when using a positive charging single-layer type electrophotosensitive material comprising a conductive substrate and a photosensitive layer formed on the conductive layer, the photosensitive layer containing a phthalocyanine compound as an electric charge generating material, characterized in that a large half-life exposure between a half-life exposure on positive charging and a half-life exposure on negative charging is four times or less as much as that of the other half-life exposure in the reversal development type image forming apparatus because of its less transfer memory.
That is, the present invention includes the following inventions:
1) A positive charging single-layer type electrophotosensitive material comprising a conductive substrate and a photosensitive layer formed on the conductive layer, the photosensitive layer containing a phthalocyanine compound as an electric charge generating material, wherein a large half-life exposure between a half-life exposure on positive charging and a half-life exposure on negative charging is four times or less as much as that of the other half-life exposure.
2) The positive charging single-layer type electrophotosensitive material according to the term 1), which contains at least a hole transferring material and an electron transferring material.
3) The positive charging single-layer type electrophotosensitive material according to the term 2) wherein the half-life exposure on positive charging is smaller than the half-life exposure on negative charging.
4) The positive charging single-layer type electrophotosensitive material according to the term 3), which contains, as the electron transferring material, at least one compound represented by the general formula (1): 
wherein R1 and R2 are the same or different and each represents a monovalent hydrocarbon group which may have a substituent, the general formula (2): 
wherein R3, R4, R6 and R6 are the same or different and each represents an alkyl group or an aryl group, or the general formula (3): 
wherein R7 and R8 are the same or different and each represents a monovalent hydrocarbon group which may have a substituent, or the general formula (4): 
wherein R9 and R10 are the same or different and each represents an alkyl group, a halogenated alkyl group, an aryl group, an aralkyl group, an alkoxy group, an aryloxy group, an aralkyloxy group, an acyl group, an alkoxycarbonyl group, an aryloxycarbonyl group, an aralkyloxycarbonyl group, or a nitro group, and n represents an integer of 0 to 3.
5) The positive charging single-layer type electrophotosensitive material according to the term 3), which contains, as the hole transferring material, at least one compound represented by the general formula (5): 
wherein R11 and R13 are the same or different and each represents an alkyl group which may have a substituent, an aryl group which may have a substituent, or an aralkyl or alkoxy group which may have a substituent, and R12 and R14 are the same or different and each represents a hydrogen atom, or an alkyl or alkoxy group which may have a substituent, provided that R12 and R14 are hydrogen atoms when the substitution position of R12 and R14 is para-position, or the general formula (6): 
wherein R15 and R17 represent the same or different alkyl group, and R16 and R18 are the same or different and each represents a hydrogen atom or an alkyl group.
6) The positive charging single-layer type electrophotosensitive material according to the term 3), which contains a terphenyl compound represented by the general formula (7): 
wherein R19, R20 and R21 are the same or different and each represents a hydrogen atom, or an alkyl, aryl or amino group which may have a substituent.
7) The single-layer type electrophotosensitive material according to the term 3), which contains the electron transferring material represented by the general formula (1), the hole transferring material represented by the general formula (6), and the terphenyl compound represented by the general formula (7).
8) The positive charging single-layer type electrophotosensitive material according to the term 3), which contains, as a binder resin, a bisphenol Z polycarbonate resin having a weight-average molecular weight of 10,000 to 400,000.
9) A positive charging reversal development type digital image forming apparatus, comprising the positive charging electrophotosensitive material of the term 1), said reversal development type digital image forming apparatus being provided with at least a main charging step, an exposure step, a developing step, a transferring step, a charge neutralizing step and a cleaning step in the forward direction of the electrophotosensitive material, wherein the voltage to be applied in the transferring step has a polarity reverse to that of the voltage to be applied in the charging step.
10) A positive charging reversal development type digital image forming apparatus, comprising the positive charging electrophotosensitive material of the term 3), said reversal development type digital image forming apparatus being provided with at least a main charging step, an exposure step, a developing step, a transferring step, a charge neutralizing step and a cleaning step in the forward direction of the electrophotosensitive material, wherein the voltage to be applied in the transferring step has a negative polarity and the voltage to be applied in the charging step has a positive polarity.
In the single-layer type electrophotosensitive material of the present invention, a larger half-life exposure among half-life exposures on positive and negative charging is four times or less as much as that of the other half-life exposure.
In case of the positive charging single-layer type electrophotosensitive material, although the half-life exposure on positive charging is smaller than the half-life exposure on negative charging (sensitivity is better), when the half-life exposure on positive charging is four times or less as much as that on negative charging, electrons remained on the surface of the photosensitive material in the transferring step are sufficiently erased in the charge neutralizing step as described above. In other words, since transportation balance of electrons and holes in the photosensitive layer becomes better with the decrease of a difference between the photosensitivity on positive charging and photosensitivity on negative charging, a small influence of space charges is exerted, thereby reducing the transfer memory.
Even when using the electrophotosensitive material of the present invention in a reversal development type image forming apparatus, it causes less transfer memory and no transfer memory image. Since the positive charging single-layer type electrophotosensitive material of the present invention is superior in resistance to gases such as ozone, NOx and the like, image fog and black streaking do not occur even when using in a large size image forming apparatus where a large amount of gases are evolved.